Upon some additional internet research I think I answered my own questions. Shaw and Tenney has a great blog post about setting up canoes for rowing with a removable rowing seat that, conviently, they sell.

As far as the thwart goes, I think I will use through bolts and wing nuts or T nuts to attach the twart rather than permanently epoxy the thwart to the gunwales. That way I can remove it for rowing and reattach for paddling. If the boat feels to fragile without the thwart I'll have to make one or two more to attach during rowing using the same through bolt setup. 

John Harris has a great piece on rowing geometry here on the website somewhere.  If you aren't able to find it, sing out and I'll try to winkle it out and share a pointer here.

I've thought about this sort of business myself.  I like to row, I enjoy rowing my PMD, and used to row my 21' Sea Pearl cat ketch extensively.  I, too, have wondered about rowing a Sassafras 16.  What I would really like to see from CLC is a lapstitch plywood version of a light Guideboat, something of about the size and general form of the 12' Vermont Packboat from Adirondack Guideboat, see here:

https://adirondack-guide-boat.com/easy-to-learn-row-boat-perfect-for-solo-fishing/

But, alas, nothing like that is currently envisioned by CLC.  What I'm imagining might look something like a Sassafras 12 with an additional strake and widened to Guideboat proportion beam, or about the beam of a Sassafras 16 on a shorter length, with the interior designed around a good solo rowing station with paddling as a secondary consideration, as with Guideboats using a "sneak paddle" as I believe they refer to it--something to move the boat along in places too narrow to spread the oars.

First problem in trying to use a Sassafras 16 for rowing figuring out how to get the structure rigid enough to handle the considerable forces concentrated at the oarlocks.  Secondly, the existing seats are too high relative to the gun'l's for good rowing geometry.  You'd need a lower seat, closer to amidships than the bow paddling seat, methinks, and, as you've noted, there's the midship cross member right there in the way.  It might be doable.  It'd be an interesting project.

.....Michael

   Michael,

Thanks for the reply and advice. I was able to find the article on rowing geometry here. That is very helpful. It is a great point you make about making sure the boat can handle rowing forces (even fixed seat). I'm wondering if it is even worth pursuing or if it is a fools errand trying to make this boat into something that it is not.  

   Interestingly, the rowing geometry article references a Whitehall that conveniently has nearly identical freeboard and beam measurements as the Sassafras 16. This makes “designing” a rowing setup quite easy. The Shaw and Tenney fixed seat rowing rig has a height of 7” which, as the article suggests would put the bottom of the oarlocks 7” from the top of the seat. On the Sassafras that works out perfectly with the existing gunwales, especially if I use the CLC Oarlock risers. The article suggests 7’ oars for the Sassafras dimensions. (777 nice)

I don’t know anything about boats or boat design so I may be missing something, but it seems the sassafras is a good candidate for casual fix seat rowing (the alternative being building and storing a 2nd boat), the only problem to solve is the thwart position. I have seen an “outrigger” design from Cape Falcon Kayaks, here, that is dropped onto a canoe for rowing. It is basically a semicircle that the rower faces that attaches to the gunwale and holds the oarlocks while also functioning as a thwart. If I planned carefully I could make the Sassafras thwart removable with bolts and T nuts and drop the semicircle outrigger into the same holes for rowing. 

   For some reason that link was blocked. Here is a link to the Cape Falcon Kayak Youtube video about the outrigger. Skip to 1:10 into the video to see the outrigger.